Field of the Invention
The present disclosure relates to a radiation imaging system, a radiation imaging apparatus, and a method of controlling the same.
Description of the Related Art
There has been a flat panel type radiation imaging apparatus provided with a sensor array formed by two-dimensionally arraying a plurality of pixels. Each pixel used for such a radiation imaging apparatus generally includes a conversion element which is obtained by forming a film on a glass substrate by using amorphous silicon or polysilicon as a material and converts radiation into an electrical signal and a switch element such as a TFT for transferring the electrical signal to the outside. In general, such a radiation imaging apparatus performs read operation by transferring, to a reading apparatus, the signals converted by conversion elements upon performing matrix driving using switch elements such as TFTs.
Each conversion element on the sensor array directly or indirectly generates a signal upon being irradiated with radiation. In a sensor array configured to indirectly generate signals, the conversion element of each pixel detects visible light converted from radiation by a phosphor instead of directly detecting radiation. In a sensor array based on either the direct detection scheme or the indirect detection scheme, each pixel generates a signal with a certain level even in the total absence of radiation. In this case, this signal will be referred to as a “dark current”.
Dark currents have different characteristics in the respective pixels on the sensor array. If dark currents are superimposed on the image signals obtained by radiation irradiation, uneven offsets are added to an image, resulting in a deterioration in image quality. In order to prevent this, the radiation imaging apparatus is configured to extract dark current charges from the sensor array periodically and/or intensively immediately before radiation irradiation by using a period during which no radiation is irradiated.
In this case, when extracting a dark current, if an image signal is superimposed on the dark current, it is not possible to separate them and extract only the dark current. That is, executing dark current extraction during radiation irradiation or in the interval after radiation irradiation and before reading of an image signal will lose the image signal. Therefore, in the radiation imaging apparatus, it is necessary to perform control so as to exclusively execute dark current extraction and radiation irradiation. For this reason, a synchronization mechanism for establishing synchronization is provided between the apparatus and a radiation source.
Depending on the scheme, some conversion element needs to periodically perform reset operation as well as dark current extraction. In this case as well, reset operation leads to the loss of an image signal, and requires exclusive control with respect to radiation irradiation.